BN_bn2hex
fprintf(fp, "\tModulus: 0x%s\n", lowercase(BN_bn2hex(RSA_get0_n(key))));
fprintf(fp, "\tPublicExponent: 0x%s\n", lowercase(BN_bn2hex(RSA_get0_e(key))));
fprintf(fp, "\tPrivateExponent: 0x%s\n", lowercase(BN_bn2hex(RSA_get0_d(key))));
fprintf(fp, "\tPrime1: 0x%s\n", lowercase(BN_bn2hex(RSA_get0_p(key))));
fprintf(fp, "\tPrime2: 0x%s\n", lowercase(BN_bn2hex(RSA_get0_q(key))));
fprintf(fp, "\tExponent1: 0x%s\n", lowercase(BN_bn2hex(RSA_get0_dmp1(key))));
fprintf(fp, "\tExponent2: 0x%s\n", lowercase(BN_bn2hex(RSA_get0_dmq1(key))));
fprintf(fp, "\tCoefficient: 0x%s\n", lowercase(BN_bn2hex(RSA_get0_iqmp(key))));
xkey = BN_bn2hex(sk);
xkey = BN_bn2hex(pk);
s = BN_bn2hex(a);
char *BN_bn2hex(const BIGNUM */*a*/);